Abstract
The effects of interface roughness between donor and acceptor in a bilayer heterojunction solar cell were investigated on a polymer–polymer system based on poly(3-hexylthiophene) (P3HT) and poly(dioctylfluorene-alt-benzothiadiazole) (F8BT). Both polymers are known to reorganize into semicrystalline structures when heated above their glass-transition temperature. Here, the bilayers were thermally annealed below glass transition of the bulk polymers (≈140 °C) at temperatures of 90, 100, and 110 °C for time periods from 2 min up to 250 min. No change of crystallinity could be observed at those temperatures. However, X-ray reflectivity and device characteristics reveal a coherent trend upon heat treatment. In X-ray reflectivity investigations, an increasing interface roughness between the two polymers is observed as a function of temperature and annealing time, up to a value of 1 nm. Simultaneously, according bilayer devices show an up to 80% increase of power conversion efficiency (PCE) for short annealing periods at any of the mentioned temperatures. Together, this is in agreement with the expectations for enlargement of the interfacial area. However, for longer annealing times, a decrease of PCE is observed, despite the ongoing increase of interface roughness. The onset of decreasing PCE shifts to shorter durations the higher the annealing temperature. Both, X-ray reflectivity and device characteristics display a significant change at temperatures below the glass transition temperatures of P3HT and F8BT.
Highlights
Organic solar cells are a promising alternative to inorganic solar cells, because production costs can be reduced through different large area deposition techniques, like roll-to-roll.[1]
Thickness of the polymer single layers varied for P3HT from 39−43 nm and for F8BT from 61−64 nm, as extracted from X-ray reflectivity (XRR) fits on single layers, which led to a predicted total bilayer thickness of 100 to 107 nm
This value was confirmed in XRR by the bilayers characteristic Kiessigfringes below qz = 0.20 Å−1 as seen in Figure 2, which correspond to the total thickness of the bilayer
Summary
Organic solar cells are a promising alternative to inorganic solar cells, because production costs can be reduced through different large area deposition techniques, like roll-to-roll.[1]. The charge separation is highly influenced by properties of an interfacial area between the donor and acceptor. This circumstance is related to the short lifetime of the Frenkel excitons, which corresponds to a short diffusion length of about 10 nm. This fact makes the distribution of donor and acceptor domains in the solar cell extremely vital. Plenty of effort has been spent on the control of domain size and distribution in bulk-heterojunction blends by annealing of the organic semiconductors.[4,5] Thereby it is generally an accepted opinion that temperatures at or slightly above glass transition of the polymer are required to allow significant motion of polymer chain segments, allowing diffusion or reorientation.[6,7] Thereby it has been shown recently that the effective glass transition temperature (Tg) of polymer thin films can occasionally be slightly reduced compared to the bulk induced by free interface effects.[8]
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